According to the prior art, automatic transmissions, in particular for motor vehicles, comprise planetary gearsets which are shifted by means of frictional shifting elements such as clutches and brakes, and usually a starting element that can operate with slip and is optionally provided with a bridging clutch, such as a hydrodynamic torque converter or a hydraulic coupling.
Such an automatic transmission is known for example from DE 199 49 507 A1 by the present applicant; in this case two non-shiftable twin-range gearsets are provided on the drive input shaft, which on the output side produce two rotational speeds which, besides the speed of the driveshaft, can be optionally engaged with a shiftable double planetary gearset that acts on the drive output shaft by the selective engagement of the shifting elements used, in such manner that to shift from a gear to the respective next higher or next lower gear, of the two shifting elements actuated at the time in each case only one shifting element has to be engaged or disengaged.
In this case, if five shifting elements are used, seven forward gears can be obtained and if six shifting elements are used, nine or ten forward gears can be obtained.
Furthermore, from DE 102 13 820 A1 a multi-gear automatic transmission with eight forward gears and one reverse gear is known, which comprises a first input pathway T1 of a first gear ratio; an input pathway T2 which has a higher gear ratio than the input pathway T1; a planetary gearset of the Ravigneaux type with four elements, these four elements being a first element, a second element, a third element and a fourth element in the sequence of the elements in a rotational speed diagram; a clutch C-2 which transmits rotation of the input pathway T2 to the first element S3; a clutch C-1 which transmits rotation of the input pathway T2 to the fourth element S2; a clutch C-4 that transmits rotation of the input pathway T1 to the first element; a clutch C-3 that transmits rotation from the input pathway T1 to the second element C3; a brake B-1 which engages the fourth element; a brake B-2 which engages the second element; and an output element which is coupled to the third element S3.
In addition a 9-gear multi-speed transmission is known from DE 29 36 969 A1; this comprises eight shifting elements and four gearsets, one gearset serving as an intermediate transmission and the main transmission comprising a Simpson gearset and a further gearset that serves as a reversing transmission.
Further multi-speed transmissions are known, for example from DE 10 2005 010 210 A1 and DE 10 2006 006 637 A1 by the present applicant.
Automatically shiftable vehicle transmission of planetary design in general have often already been described in the prior art and are continually undergoing further development and improvement. Ideally, such transmissions should entail little structural complexity, in particular requiring a small number of shifting elements, and should avoid double shifts during sequential shifting operations so that during shifts in defined gear groups, in each case only one shifting element is changed.
From the not yet published DE 102008000428.3 by the present applicant a multi-speed transmission of planetary design is known, which comprises a drive input and a drive output arranged in a housing. In this known transmission there are provided at least four planetary gearsets, referred to below as the first, second, third and fourth planetary gearsets, at least eight rotary shafts—referred to as the input shaft, the output shaft and the third, fourth, fifth, sixth, seventh and eight shafts—and at least six shifting elements, including brakes and clutches, whose selective engagement produces elements, including brakes and clutches, whose selective engagement produces various gear ratios between the drive input and the drive output so that, preferably, nine forward gears and one reverse gear can be obtained.
In this case the first and second planetary gearsets, which are preferably designed as minus planetary gearsets, form a shiftable intermediate gearset whereas the third and fourth planetary gearsets form a main gearset.
In this known multi-speed transmission it is provided that the carriers of the first and second planetary gearsets are coupled with one another by the fourth shaft, which is connected to an element of the main gearset, the ring gear of the first planetary gearset is coupled with the sun gear of the second planetary gearset by the eighth shaft, which can be releasably connected by a first clutch to the drive output shaft, and the sun gear of the first planetary gearset can be coupled to the housing of the transmission by means of the third shaft via a first brake, and can be releasably connected to the drive input shaft by a second clutch, whereas the ring gear of the second planetary gearset can be coupled with a housing of the transmission by means of the fifth shaft via a second brake. Furthermore, the seventh shaft is permanently connected to an element of the main gearset and can be coupled to the housing of the transmission by a third brake, while the sixth shaft is permanently connected to at least one other element of the main gearset and can be releasably connected to the drive input shaft by means of a third clutch; the drive output shaft is permanently connected to at least one further element of the main gearset.
Preferably, in the known transmission the fourth shaft is permanently connected to the ring gear of the third planetary gearset, whereas the sixth shaft is permanently connected to the ring gear of the fourth planetary gearset and to the carrier of the third planetary gearset, and can be releasably connected to the drive input shaft by the third clutch. In addition the seventh shaft is permanently connected to the sun gears of the third and fourth planetary gearsets and can be coupled with a housing of the transmission by the third brake. In this case the drive output takes place by way of the output shaft permanently connected to the carrier of the fourth planetary gearset. In addition the third and fourth planetary gearsets can be combined or reduced to a Ravigneaux gearset with a common carrier and a common ring gear.